Safety Clearances
There are many different values used for the insulation and safety clearances used by different utilities throughout the world. These clearances have been derived in many different ways, sometimes not following any particular logic related to the operational voltages.
Table of Contents
The method adopted commonly uses a common rule based on the voltage level of the substation, but the method of application takes into account, in each case, the operating conditions of the equipment. For the purpose of this description, the distances so obtained were defined as “safety distances.” These distances should be applied as minima where they exceed any other distances or safety clearance specified for a country or utility, but only in agreement with the utility concerned.
Table
Definition
Safety Distance
The “safety distance” means the minimum distance to be maintained in air between the live piece of equipment or conductor, on the one hand, and the earth or another piece of equipment or conductor on which it is necessary to carry out work, on the other. It should be noted that these distances are in air and do not relate to insulator lengths or creepage distances.
This “safety distance” is made up of two values as follows:
Basic value
A basic value, related to the impulse withstand voltage for the substation, which determines a “guard” zone around the conducting parts.
It should be noted that the impulse voltage withstand level is chosen rather than the highest voltage on the system, because for each standard withstand level, there is a precise corresponding non-flashover distance. However, the highest voltage on the system has a number of different impulse withstand levels and hence a number of different distances for non-flashover.
Safety zone
A value which is a function of movements to be made by staff or the nature of jobs to be carried out taking into account the devices which will be used. This determines a zone called the “safety zone” within which any danger inherent in the electrical risk has been removed and safety clearance is established.
Calculation of the Basic Value
The basic value guarantees no risk of flashover under the least favorable conditions. In order to achieve this, it is selected by taking the impulse withstand level for the substation and then looking up the corresponding non-flashover distance in air.
This distance is then increased by a safety clearance factor of 5–10% to allow for a slight scatter in the positioning of equipment due to manufacturing tolerances and for slight geometrical differences in the equipment made by different manufacturers.
The safety clearance factor can only be guaranteed if the impulse voltage taken as the basis for the calculation of the basic value is never exceeded. This requires that the substation has equipment capable of limiting the incoming surges, for example, surge arresters, spark gaps, or similar devices to provide the required degree of protection.
Usually, the basic value is the design clearance chosen for phase to earth.
Determination of the Safety Zone
In order to derive the safety zone, it is necessary to add to the basic value, determined as described and a variable factor based upon:
• The height of the operators
• The nature of the work to be carried out on the equipment
• The operating procedure and taking into account the requirement for movement and access
A method of determining the mean dimensions to be used taking into account the below is given in the table
These dimensions are basically guidelines and can be increased if considered prudent. Further clearances also need to be taken into consideration.
Movement of Staff
If there are no grills or screens fitted, then the safety distance or safety clearance between the ground and the lowest live parts must take into account the free movement for the operating staff.
From consideration of the dimensions in the Figure above, this height should be equal to the basic value plus 2.25 m (this dimension corresponds to the average height of a worker with his arms outstretched). However, as the basic value for impulse withstand voltages less than 380 kV is very small, a minimum value for the safety distance or safety clearance for staff circulation of 3.0 m is adopted. Also, the distance between the base of any post insulator and the ground should not be less than 2.25 m. The insulator is considered as a live component of steadily reducing voltage, and only the lowest metal parts are at earth potential.
The distances defined are to be considered as taken from the highest point accessible without climbing in the normal course of movement (such as raised cable ducts, steps, or gratings from which switching operations are performed if these are provided).
When the dimensions mentioned above cannot be achieved, then access to the live equipment and conductors should be prevented by the provision of screens, grills, or enclosures. Examples of such screens or enclosures being:
• A guard rail or safety clearance rail 1.2 m in height separated from the equipment or conductors by a distance equal to the basic value plus at least 0.6 m
• An enclosure or grill of height 2.25 m spaced from the equipment or conductors by a distance equal to the basic value.
Movement of Vehicles
The variable part of the horizontal safety clearance or distance comprises the profile of the vehicle or machine that needs to move plus an amount of 0.7 m to allow for unavoidable inaccuracies in driving.
Work on Equipment or on a Conductor
When it is necessary to carry out work in a substation while leaving the adjacent circuits live, then the safety clearance or distance from the live equipment is derived using the same principles. It comprises a basic value plus a variable amount determined for each piece of equipment as a function of the mode of operation of the maintenance work intended and of the dimensions of the tools used.
However, this safety zone value should never be less than 3.0 m. The safety clearance or safety distance is measured from the extreme position that the live equipment or conductor may occupy to the edge of the equipment that is to be worked on. Under no circumstances should the work involve any penetration into the basic value.
In the case of routine maintenance work that does not require equipment other than light portable tools, the variable part may be determined as follows:
• 1.75 m horizontally corresponding to a man with his arms outstretched
• 1.25 m vertically above the working plane corresponding to that part of the worker passing above the plane when his arms are outstretched.
